The present invention relates to a system for evoking a desired electroencephalogram signal, i.e., a brain wave, efficiently from a human brain.
It is known in the art that brain waves, i.e., electroencephalogram (EEG) signals, originating from the brain of a human being and the physiological and psychological states of the human being are closely related to each other. For example, when a human being is in a relaxed state, alpha (.alpha.) waves in a frequency range of from about 8 to 13 Hz are dominantly produced. Beta (.beta.) waves are prevalent in an active mental state and have a frequency ranging from about 14 to 30 Hz. Brain waves generated during drowsiness and light sleep are theta (.theta.) waves in a frequency band ranging from about 4 to 7 Hz. The correlation between these brain waves having different frequency ranges and certain human activity phases indicates that the evocation of a certain brain wave through sensory stimulation is apt to put the human being in a corresponding physiological and psychological state.
Based on the analysis of the interaction between the EEG signals and physiological and psychological states, there have heretofore been proposed various systems for evoking alpha waves from the brain of a human being by giving a certain external stimulus to his body, to thereby place him in a physically and psychologically relaxed state for assisting him in lessening stresses and achieving mental concentration.
For example, Japanese Laid-Open Patent Publication No. 55-63656 discloses a biofeedback device for putting a user in a relaxed state by picking up brain waves through electrodes attached to the scalp of the user, extracting alpha waves from the brain waves, and varying sounds emitted from earphones worn by the user depending on the intensity of the alpha waves to let the user know how the alpha waves are being produced based on different sounds.
A relaxation device disclosed in Japanese Laid-Open Patent Publication No. 62-87165 includes a random noise generator for generating random noise to artificially produce an alpha wave signal having fluctuating 1/f characteristics. A light source is turned on and off by the alpha wave signal to apply a photic stimulus to a user for thereby placing the user in a relaxed state.
For the user to reach a relaxed state with the biofeedback device described above, it is necessary that the user differentiate differing sounds emitted from the earphones and control his psychological state according to his own will in order to make the sound corresponding to the alpha waves most intensive. Before the biofeedback device is used most effectively, therefore, the user has to repeatedly practice the device to find out how mental concentration should be effected to produce intensive alpha waves, so that the psychological state can freely be changed to some extent according to the user's own will. A person who uses the biofeedback device may not necessarily achieve the desired result, and hence the effectiveness of the device may vary from user to user.
The relaxation device disclosed in the latter publication employs an artificial alpha wave signal which has no direct relationship whatsoever to the brain waves of a user for evoking the brain waves. Accordingly, the evoked EEG responses are not constant and tend to differ from one user to another.